1. Field of the Invention
This invention relates to a method for enhancing the rate of evaporation of material from a laser-heated target. More particularly, it relates to the use of a target which is coated with a layer of condensed substantially inert gas.
2. Description of the Prior Art
The preparation of high quality thin films from a variety of materials has become of substantial importance in a variety of technologies. For example, semiconductor films deposited on various substrates are utilized in the fabrication of diodes, photovoltaic cells, transistors, and other electronic devices. Similarly, thin films of various insulators and conductors are also utilized in the manufacture of electronic devices and solar cells. For example, metal-insulator-semiconductor solar cells comprise three layers, wherein the top layer is a transparent conducting metal film, the intermediate layer is a very thin layer of an insulating material such as silicon dioxide, and the bottom layer is a semiconductor such as silicon.
A large number of techniques have been developed for the deposition of thin films on a variety of substrates. Included in the various techniques which are currently available for the preparation of films are thermal evaporation from resistance, electron-beam, rf- or laser-heated sources, molecular beam epitaxy, ion implantation, plasma spraying, glow discharge sputter deposition, ion beam deposition, and chemical vapor deposition. Many of the more important methods of film deposition are summarized in "Thin Film Processes," J. L. Vossen and W. Kern, Ed., Academic Press, New York, N.Y., 1978.
The preparation of films by evaporation from a laser-heated source offers several advantages over many of the alternative methods. For example, the laser beam can be controlled in such a manner that only the source material is heated. Accordingly, this technique has the advantage of being extremely clean and does not ordinarily result in the incorporation of unwanted impurities into the resulting film. In contrast, thermal evaporation from resistance or rf-heated sources typically involves the use of a heated crucible to contain the source material and, as a consequence, there is frequently an undesired migration of impurities into the source material and film from the crucible. Laser evaporation is also a desirable method for the preparation of films because, at high power densities, laser heating typically results in congruent evaporation of source material components. Accordingly, the composition of the resulting film is usually substantially identical to that of the source material.
The laser evaporation technique has been used to produce films from a variety of materials. For example, the formation of mercury cadmium telluride (Hg.sub.1-x Cd.sub.x Te) films using a Nd:yttrium aluminum garnet (Nd:YAG) laser has been described by J. T. Cheung et al., J. Vac. Sci. Technol., Vol. 21, No. 1, 1982, pp. 182-186. Similarly, M. Hanabusa et al., Appl. Phys. Lett., Vol. 39, No. 5, 1981, pp. 431-432, have described the formation of hydrogenated amorphous silicon films using a Nd:YAG laser at wavelengths of 1.06 .mu.m and 532 nm. Formation of thin films of SnO.sub.2 using a Nd:YAG laser operated at 1.06 .mu.m has also been reported by H. T. Yang et al., J. Crystal Growth, Vol. 56, 1982, pp. 429-432.
Unstable noble gas halides, such as XeF, XeCl, XeBr, KrF and ArF have found use as light emitting species in lasers since they can be easily formed in excited states by electron-beam pumping or discharge pumping of suitable gas mixtures. Such lasers, which are referred to as "excimer lasers", operate in the ultraviolet region of the spectrum. Accordingly, they represent a source of relatively high energy photons. For example, a mixture of 10% xenon, 89% argon and 1% fluorine can be pumped with 400 keV electrons to produce excited XeF which emits light of 351 nm wavelength. Similarly, ArF, KrF and XeCl can be utilized to generate light of 193 nm, 248 nm and 308 nm wavelength, respectively.
Amorphous silicon films can be obtained by the condensation of silicon vapor on a substrate whose temperature is considerably below the melting point of silicon. Such methods include thermal evaporation, cathode sputtering, and plasma deposition by decomposition of silane (SiH.sub.4) in a glow discharge. If desired, films of amorphous silicon can be converted to crystalline silicon by annealing.
Amorphous silicon films for semiconductor use are typically prepared in a manner which results in the incorporation of up to about 30 atom percent hydrogen. Accordingly, the resulting material is often referred to as a "hydrogenated amorphous silicon" or an "amorphous silicon-hydrogen alloy". The hydrogen results in valency saturation within the amorphous silicon, which is of importance for satisfactory electric and photoelectric properties because free valencies can capture the charge carriers (electrons or holes) within the material. For example, this effect of free valencies serves to reduce the lifetime of the charge carriers and, hence, the photoconductivity of the material.
For various applications, such as the manufacture of photovoltaic cells, the electrical conductivity of silicon can be modified by doping with small quantities of impurity atoms. Most commonly, boron or phosphorus atoms have been utilized as dopants. In the case of phosphorus, a pentavalent phosphorus atom is substituted for a silicon atom in tetrahedral surroundings, and four electrons of the phosphorus atom are utilized for bonding with neighboring silicon atoms. The fifth valence electron of phosphorus is only loosely bonded and can be released as a conduction electron. In the case of boron, a trivalent boron atom is similarly substituted for a silicon atom, and accepts a total of five electrons from neighboring silicon atoms to create an electron vacancy which behaves like a positive charge and contributes to the current-carrying capability as a positive hole.